Apex domains cannot have CNAME records.
Question and response
Question
dig infoserve.de. NS
Answer
infoserve.de. 5 NS dns2.infoserve.de.
infoserve.de. 5 NS dns1.infoserve.de.
Authority
The response has no authority records.
Additional
dns1.infoserve.de. 5 A 31.177.121.4
dns2.infoserve.de. 5 A 31.177.120.220
dns1.infoserve.de. 5 AAAA 2a0b:d1c0:15:105:0:0:0:53
dns2.infoserve.de. 5 AAAA 2a0b:d1c0:15:104:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0